1. Field of the Invention
This invention relates to an oxide sintered body that is used when manufacturing a low-resistant transparent conductive film using a direct-current sputtering method, and a sputtering target based on the oxide sintered body, and a method of manufacturing a transparent conductive film based on the oxide sintered body, and particularly to an oxide sintered body and sputtering target that is used for sputtering and ion plating film formation used for manufacturing a low-resistant transparent conductive film having good permeation characteristics in the infrared range.
2. Description of the Related Art
A transparent conductive film has high conductivity and high transmittance in the visible light range. Besides being used in electrodes for solar batteries, liquid-crystal elements and other kinds of light-receiving elements, transparent conductive film is also used as heat-reflecting film for automobile windows and buildings, as anti-static film and in various kinds of anti-fog transparent heating elements for refrigerated showcases.
Tin oxide (SnO2) containing antimony or fluorine as a dopant, zinc oxide (ZnO) containing aluminum or gallium as a dopant or indium oxide (In2O3—Sn) containing tin as a dopant are widely used for transparent conductive film. Particularly, indium oxide film containing tin as a dopant, or in other words In2O3—Sn film, called ITO (indium tin oxide) film is often used because a low-resistant film can easily be obtained.
Low-resistant transparent conductive film is optimally used in electrode of touch panels, solar batteries, liquid crystals, organic electroluminescence and inorganic electroluminescence.
Sputtering is often used as the method for manufacturing these transparent conductive films. The sputtering method is an effective method when forming a film using materials having low vapor pressure, or when it is necessary to precisely control the film thickness, and since operation is very simple, it is widely used industrially.
In the sputtering method, the raw material that will be the component of the objective film is used as a sputtering target. In this method, a vacuum apparatus is generally used, and after a high vacuum is reached, a noble gas such as argon gas is introduced, and under a gas pressure of approximately 10 Pa or less, the substrate is taken to be the anode and the sputtering target is taken to be the cathode, and a glow discharge is caused to occur between these to generate argon plasma, and to cause the positive argon ions in the plasma to collide with the cathode or sputtering target, and the particles of the target component that are caused to break away by this are deposited on the substrate to form a film.
The sputtering method can be classified according to the method used for generating the argon plasma; where the radio-frequency (RF) sputtering method uses radio-frequency plasma, and the direct-current (DC) sputtering method uses direct-current plasma.
Generally, the direct-current sputtering method is widely used in industry because its speed of formation for the film is faster than that of the radio-frequency sputtering method, the power-supply equipment is inexpensive and the operation for forming the film is simple. However, in the direct-current sputtering method it is necessary to use a conductive target, whereas in the radio-frequency sputtering method it is possible to form the film even with a non-conductive material.
The speed of formation of the film for the sputtering method is closely related to the chemical bonds of the target material. Sputtering is a phenomenon that occurs when positive argon ions having kinetic energy collide with the surface of the target and the matter on the surface of the target receives that energy and breaks and flies off, and the weaker the bonds are between the ions or atoms of the target material the greater the probability of the material breaking apart and flying off due to sputtering.
Looking at a solar battery that uses transparent conductive film, solar batteries comprise laminated layers of p-type and n-type semiconductors, and are classified according to the type of semiconductors. The most widely used solar batteries are those that use silicon that is safe and very abundant. Furthermore, even among solar batteries that use silicon, there are three types: single-crystal silicon, polycrystal silicon and amorphous silicon. Also, development of solar batteries called chemical-compound thin-film solar batteries, which use compound semiconductors such as CuInSe2, GsAs or CdTe, is being performed.
In any type of solar battery, there must be a transparent conductive film for the electrode on the top surface that comes in contact with the light, and ITO film or zinc oxide (ZnO) film that has been doped with aluminum or gallium have been used. The properties desired in this transparent conduction films are low resistance and high transmittance of sunlight.
In Japanese Patent Application No. 2003-324825, the inventors made it clear that an indium oxide thin film containing titanium can be used as a transparent electrode for a solar battery. Not only does this thin film have low resistance and high transmittance in the visible light range, but it also has high transmittance in the infrared light range when compared with ITO film or zinc-oxide type film. By using this kind of transparent conductive film in the electrode on the top surface of a solar battery, it is also possible to effectively use infrared energy. This titanium-added indium-oxide thin film is excellent in that it can be manufactured using the direct-current sputtering method, and can use technology that is widely used in industry.
However, when taking productivity and lowering the cost of manufacturing into consideration, in the case of performing high-speed film formation by applying high direct-current power, arcing occurs in the sputtering target used for manufacturing the titanium-added indium-oxide thin film when the high direct-current power is applied, and it was found that high-speed film formation became impossible. Arcing during film formation causes the generation of particles, which causes a decrease in product yield. Furthermore, if arcing occurs continuously, formation of the film itself is hindered.
Moreover, as the sputtering target is used in sputtering, it gradually becomes pitted, however, the fact that its sputtering characteristics are constant up until just before being all used up, is useful from the aspect of material costs. However, in the case of the sputtering targets that have been used up until now for manufacturing titanium-added indium-oxide thin film, problems occur in that nodules (black protrusions on the target surface) are formed on the sputter surface of the sputtering target as the integrated value of the applied power increases, arcing occurs, and there is a decrease in the film formation speed.
A sintered body target to which high direct-current power could be applied in this way and for which stable direct-current sputtering could be performed to the end did not actually exist.
For a sputtering target for which arcing occurs on a small scale, arcing can be avoided by using a power supply having an arcing control function. As method the for controlling arcing, there is the DC pulsing method (method of neutralizing the charge on the target by periodically stopping the negative voltage applied to the target and applying a low positive voltage during that time), and there is a method of installing an arc-blocking circuit (a circuit that detects an increase in the discharge current when arcing occurs, and stops the power supply before it can grow into full arcing, then restarts the power supply after the current flowing to the target drops sufficiently) (refer to ‘Transparent Conductive Film Technology’, Ohmsha, pg. 193 to 195). These methods belong to the direct-current sputtering method because the direct-current discharge is basically utilized in them. However, a power supply having these functions is very expensive, so there is a problem in that it causes an increase in equipment costs. Also, even when a power supply with these functions is used, it does not mean that arcing can be completely controlled.
A titanium-added indium-oxide film formed using the sputtering method has been known. For example, the oldest example is disclosed in a document by J. L. Vossen (RCA Review, 1971, Vol. 32, pg. 289 to 296). This document centers on a description of the characteristics of ITO film that is formed by RF sputtering, and gives a manufacturing example of In2O3 film to which 20 Mole % of TiO2 is added as an impurity other than tin.
However, in this structure, there is a large amount of Ti added and the form of its existence is unclear, and the electrical resistivity of the film is given as 7.5×10−1 Ωcm, which is very high. The specific resistance, surface roughness and density of the sputtering target used are not given.
Moreover, Japanese patent publication No. Tokukai Sho 59-204625 discloses a method of using a sputtering method to manufacture an indium-oxide film containing titanium on a polyethylene terephthalate film from a indium-oxide target containing 2 wt. % to 15 wt. % of titanium oxide. However, the amount of titanium added is large and the form of its existence is unclear, and the specific resistance, surface roughness and density of the oxide sintered body is not given at all, and there is no mention of high-speed film formation, the occurrence of nodules, and stable sputtering over a long period of time.